Termination of the conductor of a superconducting cable

ABSTRACT

The invention relates to a method of terminating at least one conductor ( 10 ) of a superconducting cable comprising a plurality of superconducting tapes ( 13 ), comprising the steps of associating an electrically conductive connector ( 11 ) radially at the at least one conductor ( 10 ), embedding and end of the superconducting tapes ( 13 ) in a thermosetting resin ( 14 ), embedding an end portion of the superconducting tapes ( 13 ) in a solder ( 16 ) and achieving an electric contact by the solder ( 16 ). Moreover, the invention relates to a terminated conductor of a superconducting cable, a superconducting cable, a joint between conductors of two superconducting cables, a current transmission/distribution network, and a terminator for at least one conductor of a superconducting cable that embody the above method.

[0001] The present invention relates to the termination of the conductorof a superconducting cable.

[0002] In the present description and attached claims, the expression“superconducting cable” is used to indicate a cable intended forcarrying electric current in so-called conditions of superconductivity,that is, in conditions of almost zero electrical resistance under directcurrent transport condition.

[0003] In the present description and attached claims, the expression“conductor” is used to indicate the electrically active part of asuperconducting cable, intended for carrying the phase electric currentor that of each phase of a three-phase current system (where necessary,more in particular referred to as “phase conductor”). For the sake ofbrevity, and unless otherwise indicated, the expression “conductor” isalso used to indicate the “return conductor”, that is, the electricallyactive part of a superconducting cable capable of transmitting the samequantity of electric current of the phase conductor/s associated withit, but in the reverse direction.

[0004] In the present description and attached claims, the expression“conductor termination” is used to indicate the connection to theconductor of an electrically conductive connector to allow fixing it toa second cable conductor, either superconducting or non-superconducting,or to an electrical apparatus in general, such as a transformer, anelectrical motor, etcetera. In particular, in the case of fixing to asecond conductor of superconducting cable, the termination in the abovemeaning must thus be intended as the formation of a joint between thetwo conductors. Moreover, for brevity, the electrically conductiveconnector shall sometimes be referred to as “top connector” in thefollowing description.

[0005] A similar meaning is to be given to the terms “terminatedconductor” and “terminator”.

[0006] Warm dielectric (WD) superconducting cables and cold dielectric(CD) superconducting cables are known.

[0007] A warm dielectric superconducting cable (or each phase element ofa warm dielectric three-phase cable) essentially comprises a tubularelement for supporting one or more layers of superconducting tapes, andsubstantially defining a flow channel for a cryogenic fluid, a cryostatarranged coaxially external to the layers of superconducting cables, anda dielectric arranged coaxially external to the cryostat.

[0008] In the present description and attached claims, the expression“superconducting tapes” is used to encompass both types ofsuperconducting material described hereinafter.

[0009] The expression “superconducting material” is used to indicate amaterial such as, for example, particular ceramic materials based onmixed oxides of copper such as those discussed by Cava R., J. Am. Ceram.Soc., 83 [1], 5-28 (2000). These compounds exhibit a substantially zeroresistivity below a certain temperature, defined as criticaltemperature, or Tc. For example, the critical temperature for the abovematerials ranges between about 80K (−193° C.) and about 150K (−123° C.).

[0010] The superconducting material, in particular the BSCCO material,is commonly manufactured and used in the form of single- ormulti-filament tapes wherein filaments of superconducting material areembedded in a metal matrix, usually silver, optionally added withaluminium or magnesium; or, in particular the YBCO and REBCO material ismanufactured and used in the form of a film of superconducting materialsupported by a metal tape, and optionally coated with one or more oxidelayers.

[0011] A cold dielectric superconducting cable (or each phase element ofa cold dielectric three-phase cable) essentially comprises a tubularelement for supporting one or more layers of superconducting tapes, andsubstantially defining a flow channel for a cryogenic fluid, and,arranged coaxially external to the layers of superconducting tape, in asequence: a dielectric, a return conductor, an annular flow channel forthe cryogenic fluid, and a cryostat. As an alternative, a singlecryostat is provided for all phases present in the superconductingcable.

[0012] The tubular element for supporting the layers of superconductingtapes of the phase conductor can be at least partly made of a materialexhibiting a low electrical resistance with the function of protectingthe superconducting material from overcurrent, as described for examplein the international patent application WO 00/39812 in the name of theApplicant. With the same function, in particular in the case of thereturn conductor, a screen external to the outermost layer ofsuperconducting material can be provided, for example comprising one ormore layers of conductive tapes, for example of copper.

[0013] In the present description and attached claims, the expression“cryostability device” is used to indicate such a tubular supportingelement and/or such an external screen.

[0014] The operating temperature of a superconducting cable, a term usedto indicate the temperature at which the superconducting cable transmitselectric current in superconductivity conditions, is below the criticaltemperature of the superconducting material used.

[0015] For this purpose, as said, the superconducting cable is providedwith at least one channel for the flow of a cryogenic fluid. Thecryogenic fluid generally is helium, nitrogen, hydrogen and/or argon atapplication-specific temperature and pressure.

[0016] As known from the international patent application WO 01/08234 inthe name of American Superconductor Corporation and of the Applicant,the prolonged contact of the superconducting tapes with the cryogenicfluid at the operating temperature and pressure, as well as thesubjection to thermal cycles between such operating temperature andambient temperature, can cause the infiltration of the cryogenic fluidinto the superconducting tapes, with the consequent formation of“balloons”, which cause the deterioration of the superconducting tapeperformance.

[0017] To obviate the problem, that document describes a superconductingceramic conductor for use in a cryogenic fluid, comprising a compositeceramic superconducting tape or wire and a sealing structurehermetically surrounding the outer surface of the composite ceramictape/wire. In a first embodiment, the sealing structure is metallic andin particular, it comprises rolled metallic tapes on the greater facesof the superconducting tape, and non-porous solder fillets, for exampleof Pb—Sn—Ag, Pb—Sn, Sn—Ag, In—Pb, at the side faces of thesuperconducting tape. As an alternative, the solder can includedispersions of metallic fibres or particles in an epoxy resin.

[0018] In a different embodiment, the sealing structure comprises apolymer layer with optional metal elements dispersed therein,surrounding the outer surface of the superconducting tape or wire. Thesuperconducting tape ends can be encapsulated through solder orsilicone.

[0019] The Applicant has perceived that the problem of the cryogenicfluid infiltration occurs to a greater extent at the ends of thesuperconducting tapes, as when a conductor is cut to size uponinstallation, since the superconducting material is directly exposed atthe cross section or end.

[0020] In the following description and attached claims, the expression“end of a superconducting tape” is used to indicate a longitudinal partof a superconducting tape immediately adjacent to and comprising itsexposed cross section.

[0021] More generally, the Applicant has perceived that the terminationof a conductor of a superconducting cable must meet the two requirementsof ensuring the sealing of the ends of the superconducting tapes of theconductor against the diffusion of the cryogenic fluid, and of ensuringa good electric contact between the conductor and the top connectorand/or between the conductor and that of a second superconducting cable,retaining the above properties following thermal cycles between ambienttemperature and operating temperature.

[0022] Moreover, the Applicant has perceived that the use of only aplastic material or of only a solder is not sufficient to meet the aboverequirements because in the first case there is not sufficient electriccontact and in the second case, the amount of solder needed is such asto practically imply such a high porosity of the solder as to allow theformation of micro-channels within which the cryogenic fluid infiltratesinto the superconducting tapes.

[0023] In a first aspect, the present invention relates to a method ofterminating at least one conductor of superconducting cable comprising aplurality of superconducting tapes, comprising the steps of:

[0024] a) associating an electrically conductive connector radially atthe at least one conductor,

[0025] b) embedding an end of the superconducting tapes in athermosetting resin,

[0026] c) embedding an end portion of the superconducting tapes in asolder, and

[0027] d) achieving an electric contact by the solder.

[0028] In the present description and attached claims, the expression“radially at the conductor” is used to indicate an inside or outsideposition with respect to the conductor.

[0029] In the following description and attached claims, the expression“end portion of a superconducting tape” is used to indicate alongitudinal part of superconducting tape adjacent to its end as definedabove.

[0030] In some embodiments, the step c) of embedding an end portion ofthe superconducting tapes in a solder is carried out by embedding theend portions of at least all superconducting tapes of a same conductorin a common bulk of solder.

[0031] If there are two conductors, the solder bulk can therefore becommon to the superconducting tapes of both conductors, or two solderbulks can be made, each common to the superconducting tapes of arespective conductor.

[0032] More in particular, the step c) of embedding an end portion ofthe superconducting tapes in a solder is carried out by providing atleast one sleeve surrounding an end portion of conductor and filling thesleeve with solder.

[0033] In addition, step b) of embedding an end of the superconductingtapes in a thermosetting resin can be carried out by embedding the endsof at least all superconducting tapes of a same conductor in a commonbulk of thermosetting resin.

[0034] If there are two conductors, the thermosetting resin bulk cantherefore be common to the superconducting tapes of both conductors, ortwo thermosetting resin bulks can be made, each common to thesuperconducting tapes of a respective conductor.

[0035] More in particular, the step b) of embedding an end of thesuperconducting tapes in a thermosetting resin can be carried out byproviding at least one collar surrounding at least one end of conductorand filling the collar with the thermosetting resin.

[0036] In particularly preferred embodiments, the step d) of achievingan electric contact by the solder and the step c) of embedding an endportion of the superconducting tapes in a solder are carried out byembedding the end portions of at least all superconducting tapes of asame conductor and a corresponding portion of the connector in a commonbulk of solder.

[0037] In alternative embodiments, step b) of embedding an end of thesuperconducting tapes in a thermosetting resin is carried out byembedding at most individually said end of each superconducting tape ina respective bulk of thermosetting resin.

[0038] If there are two conductors, each thermosetting resin bulk cantherefore be intended to have the end of a single superconducting tapeembedded, or the ends of a pair of a superconducting tape of the firstconductor and of a superconducting tape of the second conductor.

[0039] More in particular, the step b) of embedding an end of thesuperconducting tapes in a thermosetting resin is carried out byintroducing at most individually the free end of each superconductingtape in a respective cap and filling at least partly each cap with thethermosetting resin.

[0040] In the following description and attached claims, the expression“free end of a superconducting tape” is used to indicate a longitudinalpart of superconducting tape adjacent to and extending from its exposedcross section. Said free end can comprise all or part of the end, all orpart of the end portion, and also a part upstream of the end portion.

[0041] In the present description and attached claims, the expression“cap” is used to indicate an element having a hole intended for looselyreceiving the free end of at least one superconducting tape, wherein thehole can be a through or a blind hole, optionally provided with one ormore leaks.

[0042] Moreover, preferably, the step c) of embedding an end portion ofthe superconducting tapes in a solder is carried out by embedding atmost individually the end portion of each superconducting tape in arespective bulk of solder.

[0043] If there are two conductors, each bulk of solder can therefore beintended to have the end of a single superconducting tape embedded, orthe ends of a pair of a superconducting tape of the first conductor andof a superconducting tape of the second conductor.

[0044] In some embodiments, there is a step e) of arranging at mostindividually a free end of each superconducting tape in a respectiveelectrically conductive cap, step b) is carried out by filling a firstpart of each cap with the thermosetting resin, and step c) is carriedout by filling at least a second part of each cap with the solder.

[0045] In this case, step d) of achieving an electric contact by thesolder can be carried out by fixing each cap in contact with theconnector.

[0046] In an alternative, step d) of achieving an electric contact bythe solder comprises providing a respective electrically conductive tapeextending from each cap.

[0047] Preferably, moreover, there is a step f) of achieving an electriccontact between the connector and a cryostability device associated tothe superconducting tapes.

[0048] In particular in the case of a phase conductor, the cryostabilitydevice is a tubular, at least partly conductive element supporting thesuperconducting tapes, and step f) of achieving an electric contactbetween the connector and the cryostability device, and step c) ofembedding an end portion of the superconducting tapes in a solder arecarried out by embedding the end portions of at least allsuperconducting tapes of a same conductor and a corresponding portion ofthe tubular element in a common bulk of solder.

[0049] In particular in the case of a return conductor, thecryostability device is an at least partly conductive screen coaxiallyexternal to the superconducting tapes, and step f) of achieving anelectric contact between the connector and the cryostability device iscarried out by fixing a portion of the screen in contact with theconnector.

[0050] Solders useful for the purposes of the present invention have amelting point lower than a temperature damaging the superconductingtapes.

[0051] Preferably, the solder is a Sn—Pb—Bi alloy.

[0052] Even more preferably, the solder is an alloy comprising 43% Sn,43% Pb and 14% Bi.

[0053] Preferably, the thermosetting resin is an epoxy or siliconeresin.

[0054] Preferably, moreover, the thermosetting resin is added with ahardener.

[0055] Even more preferably, the thermosetting resin is that availableunder the trademark Araldite® from Ciba Specialites Chimiques SA,Rueil-Malmaison Cedex, France. For example, the thermosetting resin isAraldite® added with HY 951 as a hardener.

[0056] Preferably, moreover, the thermosetting resin is added with amineral filler.

[0057] The mineral filler preferably is quartz or aluminium oxide.

[0058] More preferably, the thermosetting resin is Araldite® added withquartz or aluminium oxide as a mineral filler.

[0059] Preferably, moreover, the superconducting material of eachsuperconducting tape is an oxide of bismuth, lead, strontium, calcium,and copper (BSCCO).

[0060] Preferably, moreover, each superconducting tape comprises ahermetically sealing structure surrounding its outside surface along itslength, for example as described in the above-mentioned internationalpatent application WO 01/08234.

[0061] In a second aspect thereof, the present invention relates to aterminated conductor of a superconducting cable comprising a pluralityof superconducting tapes and an electrically conductive connectorassociated to said superconducting cable radially at said conductor,wherein ends of said superconducting tapes are embedded in athermosetting resin and end portions of said superconducting tapes areembedded in a solder, the solder being in electric contact with theconnector.

[0062] In a third aspect thereof, the present invention relates to asuperconducting cable comprising at least one conductor comprising aplurality of superconducting tapes and a respective electricallyconductive connector associated radially at said at least one conductor,wherein ends of said superconducting tapes are embedded in athermosetting resin and end portions of said superconducting tapes areembedded in a solder, an electric contact being achieved by the solder.

[0063] In a fourth aspect thereof, the present invention relates to ajoint between conductors of two superconducting cables, each conductorcomprising a plurality of superconducting tapes, the joint comprising anelectrically conductive connector butt-coupling the conductors of thetwo superconducting cables, wherein ends of the superconducting tapesare embedded in a thermosetting resin and end portions of thesuperconducting tapes are embedded in a solder, at least one of saidthermosetting resin and said solder having the superconducting tapes ofboth said conductors embedded, and an electric contact being achieved bythe solder.

[0064] In some embodiments, the joint comprises a collar surrounding theends of the two conductors and filled with the thermosetting resin.

[0065] Preferably, moreover, the joint comprises a sleeve surroundingthe end portions of the two conductors and filled with the solder.

[0066] In a fifth aspect thereof, the present invention relates to acurrent transmission/distribution network comprising at least oneterminated conductor of a superconducting cable as described above, atleast one superconducting cable as described above, and/or at least onejoint between conductors of two superconducting cables as describedabove.

[0067] In a sixth aspect thereof, the present invention relates to aterminator for at least one conductor of superconducting cablecomprising a plurality of superconducting tapes, the terminatorcomprising:

[0068] an electrically conductive connector radially combinable to saidat least one conductor,

[0069] delimiting elements intended for containing thermosetting resinand/or solder and suitable for being associated to the at least oneconductor so that ends of the superconducting tapes are embedded in thethermosetting resin, so that end portions of the superconducting tapesare embedded in the solder, and so that an electric contact is achievedby the solder.

[0070] In an embodiment, the delimiting elements comprise a collarsuitable for surrounding the ends of at least all superconducting tapesof a same conductor and intended for containing the thermosetting resin.

[0071] As an alternative or in addition, the delimiting elementscomprise a sleeve suitable for surrounding the end portions of at leastall superconducting tapes of a same conductor and intended forcontaining the solder.

[0072] Moreover, in some embodiments, the delimiting elements comprise aplurality of electrically conductive caps, each suitable for containingthe free end of at least one respective superconducting tape and eachintended for containing a bulk of thermosetting resin.

[0073] Moreover, the electrically conductive caps can be suitable forcontaining at least one bulk of solder each.

[0074] In an embodiment, each cap comprises an electrically conductivetape extending from the end opposed to the end receiving thesuperconducting tape, and said connector comprises a first and a secondtubular element, the outside surface of the first tubular element andthe inside surface of the second tubular element being sloped withrespect to the respective axes, with angles of inclination selected sothat the first and the second tubular element realise a conical couplingsuitable for clamping the electrically conductive tapes extending fromsaid caps.

[0075] Further features and advantages of the present invention shallappear more clearly from the following detailed description of someexemplifying embodiments thereof, made hereinafter with reference to theattached drawings. In the drawings:

[0076]FIG. 1 schematically shows a longitudinal and partly sectionalview of a first embodiment of a terminated conductor of superconductingcable according to the present invention;

[0077]FIG. 2 schematically shows a longitudinal and partly sectionalview of a second embodiment of a terminated conductor of superconductingcable according to the present invention;

[0078]FIG. 3 schematically shows a longitudinal and partly sectionalview of a third embodiment of a terminated conductor of superconductingcable according to the present invention;

[0079]FIGS. 4 and 5 schematically show a detail of the embodiment ofFIG. 3, respectively in front and sectioned view;

[0080]FIG. 6 shows a longitudinal sectional view of a fourth embodimentof a terminated conductor of superconducting cable according to thepresent invention;

[0081]FIG. 7 shows a longitudinal sectional view of a fifth embodimentof a terminated conductor of superconducting cable according to thepresent invention;

[0082] FIGS. 8-14 schematically show some embodiments of joints betweenconductors of two superconducting cables according to the presentinvention.

[0083]FIG. 1 shows a first embodiment of a conductor 10 ofsuperconducting cable terminated according to the present invention.

[0084] In particular, conductor 10 is representative of a phaseconductor of a warm dielectric cable, or a phase conductor of a colddielectric cable.

[0085] The illustrated conductor 10 comprises a plurality ofsuperconducting tapes 13 wound in four layers around a tubularsupporting element 12.

[0086] The tubular supporting element 12 substantially defines a flowchannel for a cryogenic fluid.

[0087] The tubular supporting element 12 preferably consists, at leastpartly, of low electrical resistance material, in order to protect thesuperconducting tapes against overcurrent, i.e. it also serves as acryostability device. A suitable tubular supporting element 12 isdescribed in the international patent application WO 00/39812 in theApplicant's name.

[0088] The superconducting tapes 13, for example, are Bi-2223 tapes insilver matrix, preferably sealed along their length against thediffusion of the cryogenic fluid, as described in the above patentapplication WO 01/08234.

[0089] A top connector 11, made of an electrically conductive material,such as for example copper, is associated to the superconducting cableradially at conductor 10. In the particular case, top connector 11 ispartly screwed into the tubular supporting element 12.

[0090] The actual shape of top connector 11 is not important for thepurposes of the present invention. It is sufficient that it exhibitsfixing means, preferably removable, conjugated to fixing means of asecond cable conductor, either superconducting or non-superconducting,or of an electric apparatus in general, such as a transformer, anelectrical motor, etcetera.

[0091] A collar 15 is arranged coaxially external to the end ofconductor 10, i.e. to the end of the group of its superconducting tapes13. Collar 15 delimits a bulk of thermosetting resin 14, wherein theends of the superconducting tapes 13 are embedded.

[0092] Collar 15 is preferably made of an electrically conductivematerial.

[0093] The thermosetting resin 14 is preferably Araldite® added with HY951 as a hardener. Moreover, such resin is preferably added with quartzor aluminium oxide [Al₂(OH)₃] as a mineral filler.

[0094] As an alternative, other thermosetting resins can be used, inparticular other epoxy or silicone resins.

[0095] A sleeve 17 is arranged coaxially external to collar 15, i.e.around end portions of the group of its superconducting tapes 13 andaround an end portion of top connector 11.

[0096] Sleeve 17 is preferably made of an electrically conductivematerial.

[0097] Sleeve 17 delimits a bulk of solder 16, wherein the end portionsof the superconducting tapes 13 and the end portion of top connector 11are embedded.

[0098] Moreover, since solder 16 infiltrates between the superconductingtapes 13, also the part of support 12 corresponding to the end portionsof the superconducting tapes 13 is embedded in solder 16. This isparticularly advantageous if said support 12 is made of an at leastpartly metal material and serves as a cryostability device.

[0099] Solder 16 preferably is a Sn, Pb, Bi alloy, such as the DAIKO PFA140 alloy available from Indium Corporation of America, Utica, N.Y.,U.S.A., whose composition is 43% Sn %, 43% Pb and 14% Bi.

[0100] While the thermosetting resin 14 effectively proofs the ends ofthe superconducting tapes 13, an electric contact is achieved by thebulk of solder 16 between conductor 10 and top connector 11, as well asthe tubular cryostability element 12.

[0101] In other words, a current path is created in the annular ring ofsolder 16 around collar 15, which exhibits less electrical resistancethan that of the thermosetting resin 14.

[0102] The bulk of solder 16, that is, the diameter of sleeve 17 withrespect to the diameter of collar 15, are to be selected so as to meetthe opposed requirements of minimising the space occupied by theconductor termination and minimising the dissipated power at thetermination, which as known, is inversely proportional to the crosssection of solder 16. The dissipated power at the maximum operatingcurrent of the superconducting cable at the maximum operatingtemperature should preferably be less than 50 W, more preferably lessthan 20 W and even more preferably, less than 10 W.

[0103] In practice, solder 16 exhibits a thickness much larger than afew tens of micromillimetres, i.e. it is in practice too porous toprevent the formation of micro-channels inside which the cryogenic fluidcould thus infiltrate in the superconducting tapes 13 in the absence ofthe thermosetting resin 14, thus degrading their performance.

[0104] Moreover, the bulk of solder 16 guarantees the necessarymechanical constrain between conductor 10 and top connector 11.

[0105] If the conductor to be terminated is arranged in a substantiallyhorizontal position, collar 15 and sleeve 17 will exhibit a respectiveopening (not shown) to fill them with the thermosetting resin 14 andwith solder 16 respectively.

[0106] On the other hand, if the conductor to terminate is arranged in asubstantially vertical position, sleeve 17 will advantageously exhibitthe shape shown, tapered at its lower end, upstream of the termination.Moreover, in this case, collar 15 could exhibit a bottom 15 a, forexample made of a plurality of spacing rings between the layers. As analternative, the termination could be carried out by first fillingsleeve 17 only partly with solder 16, up to such a height as to leavethe ends of the superconducting tapes 13 free, then filling collar 15using the hardened solder 16 as a bottom, and afterwards, finishing tofill sleeve 17 with solder 16.

[0107] Moreover, both collar 15 and sleeve 17 could be joined with topconnector 11, for example through radial crosspieces (not shown).

[0108] As an alternative, conductor 10 can be representative of a returnconductor of a cold dielectric cable. In this case, as mentioned at thebeginning of the present description, the tubular supporting element 12of the superconducting tapes 13 is missing, the superconducting tapes 13being wound on the dielectric. Since the dielectric typically comprisesa wrap of paper tapes that could be damaged by the contact with solder16, it may be necessary to insert a tubular protective element betweenthe dielectric and the superconducting tapes 13. Such a tubularprotective element can be regarded as schematically represented in FIG.1 by the tubular supporting element 12.

[0109]FIG. 2 shows an embodiment of termination of the conductor of asuperconducting cable, which is modified with respect to thatillustrated and described with reference to FIG. 1 in the followingaspects.

[0110] Sleeve 17 is replaced by a sleeve 17 a whose inner diameteressentially corresponds to the outer diameter of collar 15 and does notextend longitudinally at the end portion of top connector 11.

[0111] Sleeve 17 a thus delimits a bulk of solder 16, wherein only theend portions of the superconducting tapes 13 are embedded, whereas theend portion of top connector 11 is not.

[0112] The electric contact between conductor 10 and top connector 11 isachieved by solder 16, sleeve 17 a and a wrap of conductor braids ortapes 18, for example of copper, between sleeve 17 a and top connector11.

[0113] Also in this embodiment, solder 16 in practice exhibits athickness of more than a few tens of micromillimetres.

[0114]FIG. 3 shows a third embodiment of a conductor 20 ofsuperconducting cable terminated according to the present invention.

[0115] In particular, conductor 20 is representative of a phaseconductor of a warm dielectric cable, or of a phase conductor of a colddielectric cable.

[0116] The illustrated conductor 20 comprises a plurality ofsuperconducting tapes 22 wound in two layers around a tubular supportingelement 21. The superconducting tapes 22 and the tubular supportingelement 21 are as described with reference to the superconducting tapes13 and to the tubular supporting element 12 respectively of theembodiment of FIG. 1.

[0117] A top connector, made of an electrically conductive material,such as for example copper, is radially associated to thesuperconducting cable at conductor 20. In the particular case, the topconnector comprises a first tubular element 23 a clamped, such as by acrimping in the region indicated with reference number 24, around thetubular supporting element 21.

[0118] In order to prevent distortions and damages to the tubularsupporting element 21, the top connector preferably comprises a secondtubular element 23 c inserted inside the tubular supporting element 21and coupled to the first tubular element, for example screwed asindicated at 25. Also in this case, the actual shape of top connector 23a-23 c is not important.

[0119] The free end of each superconducting tape 22 is loosely insertedin a respective cap 27, as can be better seen in FIGS. 4 and 5. Each cap27 is made of an electrically conductive material, for example copper.

[0120] Each cap 27 is filled with thermosetting resin 44 for a firstpart, wherein the end of the respective superconducting tape 22 isembedded.

[0121] Moreover, each cap 27 is filled with solder 46 for a second part,wherein an end portion of the respective superconducting tape 22 isembedded.

[0122] The thermosetting resin 44 and the solder 46 are as describedwith reference to the thermosetting resin 14 and the solder 16respectively of the embodiment of FIG. 1.

[0123] Each cap 27 preferably exhibits a hole 27 a, optionally threaded,at the opposed end of the insertion end of the respectivesuperconducting tape 22.

[0124] Referring again to FIG. 3, each cap 27 is in electric contactwith top connector 23 a-23 c, as by surface contact and fixing by arespective screw 28 to top connector 23 a-23 c.

[0125] In the particular case illustrated in FIG. 3, each cap 27 isfixed to an element 23 b of the top connector screwed outside element 23a of the top connector. Element 23 b of the top connector exhibits anoutside stepwise surface, where the steps are preferably sloped withrespect to its longitudinal axis, so that the superconducting tapes 22of the innermost layer are fixed, by the respective cap 27, at a lowerslope than the superconducting tapes of the outermost layer, so that therespective caps 27 and screws 28 do not interfere or damage thesuperconducting tapes 22 of the outermost layer. Moreover, in this way,a larger diameter is available that allows using caps larger than thesuperconducting tapes. If conductor 20 exhibits a number of layers ofsuperconducting tapes 22 other than two, element 23 b of the topconnector will exhibit a corresponding number of steps.

[0126] Elements 23 a and 23 b of the top connector can be replaced witha single element.

[0127] Also in the embodiment of FIG. 3, the thermosetting resin 44proofs the ends of the superconducting tapes 22.

[0128] The electric contact between conductor 20 and top connector 23a-23 c is achieved by solder 46, caps 27 and the contact relationbetween caps 27 and top connector 23 a-23 c.

[0129] In case the tubular element 21 is at least partly electricallyconductive for cryostability reasons, the electric contact between thetubular element 21 and top connector 23 a-23 c is provided by thecontact relationship between them.

[0130] Finally, FIG. 3 shows a wrap 25 of the superconducting tapes 22upstream of top connector 23 a-23 c.

[0131] Also in the case of the embodiment of FIG. 3, the amount ofsolder 46 needed to provide sufficient electric conductivity andsufficient mechanical constrain between each superconducting tape 22 andthe respective cap 27 is such that, in practice, it is too porous toperform the function of proofing against the cryogenic fluid.

[0132] Also conductor 20 of the present embodiment can alternativelyrepresent a return conductor of a cold dielectric cable, with thechanges described with reference to the embodiment of FIG. 1.

[0133] A further embodiment of termination of the conductor of asuperconducting cable is illustrated in FIG. 6. Such embodiment isparticularly advantageous in the case of a return conductor of a colddielectric cable.

[0134] The illustrated conductor 30 comprises a plurality ofsuperconducting tapes 31 wound in two layers around an insulator 35. Thesuperconducting tapes 31 are as described with reference to thesuperconducting tapes 13 of the embodiment of FIG. 1.

[0135] Moreover, there are illustrated two layers of conductive tapes33, for example of copper, wound around the superconducting tapes 31 andhaving the function of a cryostability device, as described in theinternational patent application WO 00/39812 in the Applicant's name.

[0136] The free end of each superconducting tape 31 is inserted in arespective cap 34 containing thermosetting resin 44 and solder 46analogously to what described with reference to FIGS. 3-5.

[0137] The superconducting tapes 31 are radially spaced from dielectric35, for example by thinning dielectric 35, as shown at 35 a.

[0138] A top connector comprises a first electrically conductive tubularelement 32 a, for example of copper or alloys thereof, inserted on thesuperconducting cable between dielectric 35 and the superconductingtapes 31, and a second electrically conductive tubular element 32 b, forexample of copper or alloys thereof, inserted on the superconductingcable outside the superconducting tapes 31 and the conducting tapes 33,if present.

[0139] The outside surface of the first tubular element 32 a and theinside surface of the second tubular element 32 b are sloped as regardsto the respective longitudinal axes, with inclination angles selected sothat the first and the second tubular element 32 a, 32 b of the topconnector establish a conical coupling, thus providing the necessarymechanical constrain between top connector 32 a, 32 b and thesuperconducting tapes 32 as well as the conductive tapes 33, if present.

[0140] The electric contact between conductor 30 and top connector 32a-32 b is achieved by solder 46, caps 34 and the contact relationshipbetween caps 34 and top connector 32 a-32 b.

[0141] Caps 34 can be fixed to top connector 32 a analogously to whatdescribed with reference to the embodiment of FIG. 3.

[0142]FIG. 7 shows an embodiment of termination of the conductor of asuperconducting cable that is modified with respect to that illustratedand described with reference to FIG. 6 in the following aspects.

[0143] The superconducting tapes 31 are cut upstream of top connector 32a, 32 b and inserted in respective caps 34′ similar to caps 34 of theembodiment of FIGS. 3-5, but showing a respective conductive tape 36,for example made of copper or alloys thereof, extending from the endopposed to that of insertion of the respective superconducting tape 31.Caps 34′ do not exhibit hole 27 a present in caps 34 of the embodimentof FIGS. 3-5.

[0144] In this way, the conductive tapes 36 and the optionalcryostability conductive tapes 33 are clamped by the conical couplingprovided by the two tubular elements 32 a, 32 b of the top connector,whereas the superconducting tapes 31 are not. Thus, the superconductingtapes 31 are not stressed, thus preventing any risk of damage.

[0145] The electric contact between conductor 30 and top connector 32 a,32 b is achieved by solder 46, caps 34′, conductive tapes 36 and thecontact relation between the conductive tapes 36 and top connector 32a-32 b.

[0146] In an alternative embodiment (not shown), collar 15 of theembodiment of FIGS. 1 and 2 and the respective thermosetting resin 14could be replaced with a plurality of caps similar to caps 27 of theembodiment of FIG. 3, each cap being filled with just the thermosettingresin.

[0147] FIGS. from 8 to 14 show, in a very schematic manner, someembodiments of joint between conductors of two superconducting cablesaccording to the present invention, which shall be described only intheir major features. For further details, reference shall be each timemade to the description of the relevant FIGS. from 1 to 7.

[0148] All FIGS. from 8 to 14 show a conductor 50 of a firstsuperconducting cable, comprising a plurality of superconducting tapes51 wound in more layers around a tubular supporting element 52, and aconductor 60 of a second superconducting cable, comprising a pluralityof superconducting tapes 61 wound in more layers around a tubularsupporting element 62.

[0149] Moreover, an electrically conductive connector 70 whichbutt-couples the two conductors 50, 60, for example by being inserted inthe tubular supporting elements 52, 62, is illustrated. Connector 70 isschematically indicated as single-piece, but as an alternative it cancomprise two complementary elements removably connected to one another.

[0150] In the embodiment of FIG. 8, the ends of the superconductingtapes 51 of the first conductor 50 are embedded in a first bulk 53 ofthermosetting resin contained in a first collar 54, and the ends of thesuperconducting tapes 61 of the second conductor 60 are embedded in asecond bulk 63 of thermosetting resin contained in a second collar 64.The bulks of thermosetting resin 53 and 63 are illustrated as havingalso respective portions of connector 70 embedded, but this is notnecessary.

[0151] The end portions of the superconducting tapes 51 of the firstconductor 50 and the end portions of the superconducting tapes 61 of thesecond conductor 60 are embedded in a common bulk 71 of solder containedin a common sleeve 72.

[0152] The embodiment of FIG. 9 differs from that of FIG. 8 in that theends of the superconducting tapes 51 of the first conductor 50 and theends of the superconducting tapes 61 of the second conductor 60 areembedded in a common bulk 73 of thermosetting resin contained in acommon collar 74. The bulk of thermosetting resin 73 has also thecorresponding portion of connector 70 embedded.

[0153] The embodiment of FIG. 10 differs from that of FIG. 8 in that theends of the superconducting tapes 51 of the first conductor 50 and theends of the superconducting tapes 61 of the second conductor 60 areembedded in respective bulks of thermosetting resin contained inrespective caps 55, 65.

[0154] In the embodiment of FIG. 11, the ends of the superconductingtapes 51 of the first conductor 50 and the ends of the superconductingtapes 61 of the second conductor 60 are embedded in a common bulk 73 ofthermosetting resin contained in a common collar 74. The bulk ofthermosetting resin 73 has also the matching portion of connector 70embedded.

[0155] The end portions of the superconducting tapes 51 of the firstconductor 50 are embedded in a first bulk 56 of solder contained in afirst sleeve 57 and the ends of the superconducting tapes 61 of thesecond conductor 60 are embedded in a second bulk 66 of solder containedin a second sleeve 67. A wrap of conductive tapes or braids 75, forexample of copper, electrically connects sleeves 57, 67.

[0156] In the embodiment of FIG. 12, the free ends of thesuperconducting tapes 51 of the first conductor 50 and the free ends ofthe superconducting tapes 61 of the second conductor 60 are inserted inpairs in common caps 76. Caps 76, of course provided with through holes,are filled, in a central part, with thermosetting resin, wherein theends of both superconducting tapes 51, 61 are embedded, and, in twoparts adjacent to the central part, with solder, wherein the endportions of each superconducting tape 51, 61 are individually embedded.

[0157] In the embodiment of FIG. 13, the free ends of thesuperconducting tapes 51 of the first conductor 50 are inserted inrespective caps 55 and the free ends of the superconducting tapes 61 ofthe second conductor 60 are inserted in respective caps 65. Caps 55 and65 are each filled with a bulk of thermosetting resin at the end of therespective superconducting tape 51, 61, and with a bulk of solder at theend portion of the respective superconducting tape 51, 61. Moreover,caps 55 and 65 are connected in pairs by a respective common conductivetape 77.

[0158] In the embodiment of FIG. 14, the free ends of thesuperconducting tapes 51 of the first conductor 50 are inserted inrespective caps 55 and the free ends of the superconducting tapes 61 ofthe second conductor 60 are inserted in respective caps 65. Caps 55 and65 are each filled with a bulk of thermosetting resin at the end of therespective superconducting tape 51, 61.

[0159] The end portions of the superconducting tapes 51 of the firstconductor 50 are embedded in a first bulk 56 of solder contained in afirst sleeve 57 and the ends of the superconducting tapes 61 of thesecond conductor 60 are embedded in a second bulk 66 of solder containedin a second sleeve 67. A wrap of conductive tapes or braids 75, forexample of copper, electrically connects sleeves 57, 67.

EXAMPLE 1

[0160] A conductor sample was made using Bi-2223 superconducting tapesin silver matrix for which proof against the infiltration of liquidnitrogen along their entire length had been previously checked. Theconductor sample is 10 metres long.

[0161] The sample was terminated at each end as described with referenceto FIG. 1, using Araldite® added with HY 951 and aluminium oxide, asthermosetting resin 14, and alloy DAIKO PFA 140 as solder 16.

[0162] The sample thus terminated was mounted into a cryostat andconstrained at the ends on a stiff frame, so as to simulate the tractionto which the termination is subject in a clamped-head installationconfiguration. As known, such a configuration represents the mostcritical condition of traction by shrinkage upon cooling of thesuperconducting cable.

[0163] The sample was immersed in liquid nitrogen and subjected to 10thermal cycles between ambient temperature and the operating temperatureof 77K at atmospheric pressure. Afterwards, still in clamped-headconfiguration, the sample was left immersed for 120 hours in liquidnitrogen at a pressure of about 30 bar.

[0164] At the end of the above stresses, the sample was analysed.

[0165] Thermosetting resin 14 and solder 16 did not exhibit eithercracks or fractures. The superconducting tapes 13 did not exhibit eithersurface “balloons” or other damages.

EXAMPLE 2

[0166] A sample of cold dielectric superconducting cable (free fromcryostat) was made using Bi-2223 superconducting tapes in silver matrixfor which proof against the infiltration of liquid nitrogen along theirentire length had been previously checked. The conductor cable sample is10 metres long.

[0167] The phase conductor of the sample was terminated at each end asdescribed with reference to FIG. 3, and the return conductor of thesuperconducting cable sample was terminated at each end as describedwith reference to FIG. 6.

[0168] Araldite® added with HY 951 and aluminium oxide, as thermosettingresin 44, and the above alloy DAIKO PFA 140 as solder 46 were used inall of the terminations.

[0169] The sample thus terminated was mounted into a cryostat andconstrained at the ends on a stiff frame, so as to simulate the tractionto which the termination is subject in a clamped-head installationconfiguration.

[0170] The sample was immersed in liquid nitrogen and subjected to 10thermal cycles between ambient temperature and the operating temperatureof 77K at atmospheric pressure.

[0171] Afterwards, the sample was left immersed for 120 hours in liquidnitrogen at a pressure of about 30 bar.

[0172] At the end of the above stresses, the sample was analysed.

[0173] Thermosetting resin 44 and solder 46 inside caps 27, 34 did notexhibit either cracks or fractures. The superconducting tapes 22, 32 didnot exhibit either surface “balloons” or other damages.

[0174] Moreover, the termination according to the invention meets therequirements of mechanical constrain and protection of the integrity ofthe superconducting tapes.

1. Method of terminating at least one conductor (10, 20, 30, 50, 60) ofsuperconducting cable comprising a plurality of superconducting tapes(13, 22, 31, 51, 61), comprising the steps of: a) associating anelectrically conductive connector (11, 23 a-23 c, 32 a-32 b, 70)radially at the at least one conductor (10, 20, 30, 50, 60), b)embedding an end of the superconducting tapes (13, 22, 31, 51, 61) in athermosetting resin (14, 44, 53, 63, 73), c) embedding an end portion ofthe superconducting tapes (13, 22, 31, 51, 61) in a solder (16, 46, 56,66, 71), and d) achieving an electric contact by the solder (16, 46, 56,66, 71).
 2. Method according to claim 1, characterised in that the stepc) of embedding an end portion of the superconducting tapes in a solderis carried out by embedding the end portions of at least allsuperconducting tapes (13, 22, 31, 51, 61) of a same conductor in acommon bulk of solder (16, 56, 66, 71).
 3. Method according to claim 2,characterised in that the step c) of embedding an end portion of thesuperconducting tapes in a solder is carried out by providing at leastone sleeve (17, 17 a, 57, 67, 72) surrounding an end portion ofconductor and filling the sleeve with the solder (16, 56, 66, 71). 4.Method according to claim 1, characterised in that the step b) ofembedding an end of the superconducting tapes in a thermosetting resinis carried out by embedding the ends of at least all superconductingtapes (13, 22, 51, 61) of a same conductor in a common bulk ofthermosetting resin (14, 53, 63, 73).
 5. Method according to claim 4,characterised in that the step b) of embedding an end of thesuperconducting tapes in a thermosetting resin is carried out byproviding at least one collar (15, 54, 64, 74) surrounding at least oneend of conductor and filling the collar with the thermosetting resin(14, 53, 63, 73).
 6. Method according to claim 1, characterised in thatthe step d) of achieving an electric contact by the solder and the stepc) of embedding an end portion of the superconducting tapes in a solderare carried out by embedding the end portions of at least allsuperconducting tapes (13, 51, 61) of a same conductor and acorresponding portion of the connector in a common bulk of solder (16,56, 66, 71).
 7. Method according to clam 1, characterised in that stepb) of embedding an end of the superconducting tapes in a thermosettingresin is carried out by embedding at most individually said end of eachsuperconducting tape (22, 33, 51, 61) in a respective bulk ofthermosetting resin (44).
 8. Method according to clam 7, characterisedin that step b) of embedding an end of the superconducting tapes in athermosetting resin is carried out by inserting at most individually thefree end of each superconducting tape (22, 33, 51, 61) in a respectivecap (27, 34, 34′, 55, 65, 76) and filling at least partly each cap withthe thermosetting resin (44).
 9. Method according to clam 1 or 7,characterised in that step c) of embedding an end portion of thesuperconducting tapes in a solder is carried out by embedding at mostindividually the end portion of each superconducting tape (22, 33, 51,61) in a respective bulk of solder (46).
 10. Method according to claim1, characterised by a step e) of arranging at most individually a freeend of each superconducting tape (22, 33, 51, 61) in a respectiveelectrically conductive cap (27, 34, 34′, 55, 65, 76), in that said stepb) is carried out by filling a first part of each cap (27, 34, 34′, 55,65, 76) with the thermosetting resin (44), and in that said step c) iscarried out by filling at least a second part of each cap (27, 34, 34′,55, 65, 76) with the solder (46).
 11. Method according to claim 10,characterised in that step d) of achieving an electric contact by thesolder is carried out by fixing each cap (27, 34, 34′, 55, 65, 76) incontact with the connector (23 a-23 c, 32 a-32 b).
 12. Method accordingto claim 10, characterised in that step d) of achieving an electriccontact by the solder comprises providing a respective electricallyconductive tape (36, 77) extending from each cap (34′, 55, 65). 13.Method according to claim 1, characterised by a step of: f) achieving anelectric contact between the connector and a cryostability deviceassociated to the superconducting tapes.
 14. Method according to claim13, characterised in that the cryostability device is a tubular, atleast partly conductive element supporting the superconducting tapes,and in that step f) of achieving an electric contact between theconnector and the cryostability device, and step c) of embedding an endportion of the superconducting tapes in a solder are carried out byembedding the end portions of at least all superconducting tapes (13,51, 61) of a same conductor and a corresponding portion of the tubularelement in a common bulk of solder (16, 56, 66, 71).
 15. Methodaccording to claim 13, characterised in that the cryostability device isan at least partly conductive screen coaxially external to thesuperconducting tapes, and in that step f) of achieving an electriccontact between the connector and the cryostability device is carriedout by fixing a portion of the screen (33) in contact with the connector(32 a-32 b).
 16. Method according to claim 1, characterised in that thesolder has a melting point lower than a temperature damaging thesuperconducting tapes.
 17. Method according to claim 16, characterisedin that the solder is a Sn—Pb—Bi alloy.
 18. Method according to claim17, characterised in that the solder is an alloy comprising 43% Sn, 43%Pb and 14% Bi.
 19. Method according to claim 1, characterised in thatthe thermosetting resin is a resin selected from the group comprisingepoxy and silicone resins.
 20. Method according to claim 19,characterised in that the epoxy resin is Araldite®.
 21. Method accordingto claim 1, characterised in that the thermosetting resin is added witha hardener.
 22. Method according to claim 21, characterised in that thethermosetting resin is Araldite® added with HY 951 as a hardener. 23.Method according to claim 1, characterised in that the thermosettingresin is added with a mineral filler.
 24. Method according to claim 23,characterised in that the mineral filler is selected from the groupcomprising quartz and aluminium oxide.
 25. Method according to claim 24,characterised in that the thermosetting resin is Araldite® added withaluminium oxide as a mineral filler.
 26. Method according to claim 1,characterised in that the superconducting material of eachsuperconducting tape is an oxide of bismuth, lead, strontium, calcium,and copper (BSCCO).
 27. Method according to claim 1, characterised inthat each superconducting tape comprises a sealing structurehermetically surrounding its outside surface along its entire length.28. Terminated conductor (10, 20, 30) of a superconducting cablecomprising a plurality of superconducting tapes (13, 22, 31) and anelectrically conductive connector (11, 23-23 c, 32 a-32 b) associated tosaid superconducting cable radially at said conductor, wherein ends ofsaid superconducting tapes are embedded in a thermosetting resin (14,44) and end portions of said superconducting tapes are embedded in asolder (16, 46), the solder being in electric contact with theconnector.
 29. Superconducting cable comprising at least one conductor(10, 20, 30) comprising a plurality of superconducting tapes (13, 22,31) and a respective electrically conductive connector (11, 23-23 c, 32a-32 b) associated radially at said at least one conductor, wherein endsof said superconducting tapes are embedded in a thermosetting resin (14,44) and end portions of said superconducting tapes are embedded in asolder (16, 46), an electric contact being achieved by the solder. 30.Joint between conductors (50, 60) of two superconducting cables, eachconductor comprising a plurality of superconducting tapes (51, 61), thejoint comprising an electrically conductive connector (70) butt-couplingthe conductors of the two superconducting cables, wherein ends of thesuperconducting tapes are embedded in a thermosetting resin (44, 53, 63,73) and end portions of the superconducting tapes are embedded in asolder (46, 56, 66, 71), at least one of said thermosetting resin andsaid solder having the superconducting tapes of both said conductorsembedded, and an electric contact being achieved by the solder. 31.Joint according to claim 30, characterised in that it comprises a collar(74) surrounding the ends of the two conductors and filled with thethermosetting resin (73).
 32. Joint according to claim 30, characterisedin that it comprises a sleeve (72) surrounding the end portions of thetwo conductors and filled with the solder (71).
 33. Currenttransmission/distribution network comprising at least one terminatedconductor of a superconducting cable according to claim 28, at least onesuperconducting cable according to claim 29, and/or at least one jointbetween conductors of two superconducting cables according to claim 30.34. Terminator for at least one conductor (10, 20, 30, 50, 60) ofsuperconducting cable comprising a plurality of superconducting tapes(13, 22, 31, 51, 61), the terminator comprising an electricallyconductive connector (11, 23 a-23 c, 32 a-32 b, 70) combinable radiallyat said at least one conductor, and delimiting elements (27, 34, 34′,55, 65, 76, 15, 54, 64, 74, 17, 17 a, 57, 67, 72) intended forcontaining thermosetting resin (14, 44, 53, 63, 73) and/or solder (16,46, 56, 66, 71) and suitable for being associated to the at least oneconductor so that the ends of the superconducting tapes are embedded inthe thermosetting resin, so that end portions of the superconductingtapes are embedded in the solder and so that an electric contact isachieved by the solder.
 35. Terminator according to claim 34,characterised in that said delimiting elements comprise a collar (15,54, 64, 74) suitable for surrounding the ends of at least allsuperconducting tapes (13, 51, 61) of a same conductor and intended forcontaining the thermosetting resin (14, 53, 63, 73).
 36. Terminatoraccording to claim 34, characterised in that said delimiting elementscomprise a sleeve (17, 17 a, 57, 67, 72) suitable for surrounding theend portions of at least all superconducting tapes (13, 51, 61) of asame conductor and intended for containing the solder (16, 56, 66, 71).37. Terminator according to claim 34, characterised in that saiddelimiting elements comprise a plurality of electrically conductive caps(27, 34, 34′, 55, 65, 76), each suitable for containing the free end ofat least one respective superconducting tape (22, 31, 51, 61) and eachintended for containing a bulk of thermosetting resin (44). 38.Terminator according to claim 37, characterised in that saidelectrically conductive caps (27, 34, 34′, 55, 65, 76) are suitable forcontaining also at least one bulk of solder (46) each.
 39. Terminatoraccording to claim 37, characterised in that each of said caps (34′)comprises an electrically conductive tape (36) extending from the endopposed to the end receiving the superconducting tape (31), and in thatsaid connector comprises a first and a second tubular element (32 a, 32b), the outside surface of the first tubular element (32 a) and theinside surface of the second tubular element (32 b) being sloped withrespect to the respective axes, with inclination angles selected so thatthe first and the second tubular element (32 a, 32 b) establish aconical coupling suitable for clamping the electrically conductive tapes(36) extending from said caps (34′).